1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more specifically, to an improvement of a method of forming isolation regions for electrically separating circuit elements in semiconductor devices.
2. Description of the Background Art
In a semiconductor integrated circuit including an FET (Field Effect Transistor), a bipolar transistor, a diode, a capacitor, a register and so on formed on a silicon substrate, means for electrically separating these circuit elements is necessary. Generally, a isolation region of a silicon oxide film, which is a highly dielectric material, is utilized as the separating means. In order to form the isolation region of the silicon oxide film, known LOCOS (Local Oxidation of Silicon) method is generally used.
FIGS. 1A to 1C are cross sectional views showing one example of steps for forming the isolation regions by the conventional LOCOS method.
Referring to FIG. 1A, an underlying silicon oxide film 12 is formed on a silicon substrate 11 to have the thickness of about 500 .ANG. by thermal oxidation. On the underlying silicon oxide film 12, a silicon nitride film 13 having the thickness of about 1000 .ANG. is deposited by, for example, CVD (Chemical Vapor Deposition). The underlying oxide film 12 is provided for releasing stress applied by silicon nitride film 13 to silicon substrate 11.
Referring to FIGS. 1A and 1B, silicon nitride film 13 is patterned by etching using photolithography such that silicon nitride film 13a is left only on a circuit element region 19 of silicon substrate 11 at which a circuit element is to be formed. More specifically, at regions where isolation regions are to be formed, underlying silicon oxide film 12 is exposed.
Referring to FIGS. 1B and 1C, silicon substrate 11 is thermally oxidized in an oxygen atmosphere using the patterned silicon nitride film 13a as a mask, and isolation regions 14 of silicon oxide film having the thickness of about 5000 .ANG. are formed.
Thereafter, the pattern 13a of silicon nitride film is removed, and a circuit element is formed by a known method in the circuit element region 19a surrounded by the isolation regions 14.
However, as shown in FIG. 1C, in the LOCOS method, reaction of oxidization of silicon substrate 11 proceeds not only in the vertical direction but also to regions below edges of silicon nitride film 13a, and therefore, bird's beak portions 15 are unavoidably formed at the isolation regions 14. The circuit element region 19a after the formation of isolation regions 14 becomes undesirably narrow, due to these bird's beaks 15. In other words, the bird's beaks 15 substantially increases areas occupied by the isolation regions 14 on silicon substrate 11. Growth of the bird's beaks 15 is not preferred in increasing the degree of integration of semiconductor integrated circuits.
A method of forming isolation regions of silicon oxide film with growth of bird's beaks suppressed has been proposed in the prior art, as shown in FIGS. 2A and 2B.
Referring to FIG. 2A, an underlying silicon oxide film 22 is formed on a silicon substrate 21. A polysilicon layer 26 is formed by vapor deposition on underlying silicon oxide film 22. A silicon nitride film 23 is formed on polysilicon layer 26.
Referring to FIGS. 2A and 2B, silicon nitride film 23 is patterned and a pattern 23a of silicon nitride film is left only on a circuit element region. Thereafter, polysilicon film 26 and silicon substrate 21 are thermally oxidized selectively using the pattern 23a of silicon nitride film as a mask, and thus isolation regions 24 of silicon oxide film are formed.
Bird's beaks 25 are generated unavoidably in this method also. However, compared with the bird's beaks 15 of FIG. 1C, bird's beaks 25 of FIG. 2B are considerably smaller. In order to suppress growth of bird's beaks 25, it is preferred to form thick polysilicon film 26. However, if polysilicon film 26 is made thick, abnormal projecting portions 27 of isolation regions 24 grow adjacent to the upper portions of bird's beaks 25. These projections 27 have undesired influences to the following processes. For example, in the following process, undesired material may remain in a concave between the projection 27 and the bird's beak 25.